Non-vitreous inorganic articles are becoming increasingly important in commerce as high performance materials. For example, non-vitreous ceramic fibers are finding utility not only as high temperature insulating materials, but also as reinforcing materials in composite structures, for example, in metals, glasses and ceramics. The reinforcement application requires fibers to have a high tensile strength and a high modulus of elasticity.
It is known that an oxide ceramic must be fully dense and have a polycrystalline structure if it is to achieve optimum tensile strength and modulus of elasticity (E). Whenever porosity is present, reduced or lower tensile strengths and modulus of elasticity can be expected. To reduce porosity in inorganic materials, the process of sintering is used which is normally accompanied by growth of the crystallites. Unfortunately, large crystallites or grains have the effect of reducing the tensile strength of polycrystalline fibers. Thus, the improvement in tensile strength attributed to the reduction in porosity by sintering is partially offset by the larger crystallites which have grown during sintering. Therefore, to produce inorganic fibers with a high tensile strength and a high modulus of elasticity (E), a dense ceramic (minimum porosity) with the smallest crystallites possible is preferred.
It is known to use organic precursors to produce a second SiC phase in oxide ceramics. U.S. Pat. No. 4,010,233 discloses inorganic fibers wherein a metal oxide phase contains a second dispersed phase. In all cases, the dispersed phase is an in situ precipitation or chemical reaction product; for the examples utilizing SiC, it is obtained via chemical reaction of an organic precursor. The particle size is dependent upon the firing conditions used; for example, time, temperature and atmosphere. E values up to 269 GPa (39.times.10.sup.6 psi) are reported.
U.S. Pat. Nos. 4,298,558 and 4,314,956 disclose alkoxylated and phenoxylated methyl polysilane which are useful for the preparation of fine grained silicon carbide-containing ceramics. Pyrolysis and reaction of the ceramic precursor polymers provide the silicon carbide-containing ceramics. SUMMARY OF THE INVENTION
Briefly, the present invention provides a shaped article comprising a ceramic matrix and having therein 5 to 30 weight percent mechanically-added silicon carbide, the article having a modulus of elasticity (E) value of at least ten percent higher, preferably 25 percent, more preferably 50 percent higher than that inherent in the fully dense host ceramic matrix. The silicon carbide is added to the ceramic matrix prior to densification as crystalline particles having an average diameter 0.1 micrometers or less.
Preferably, the surface of the shaped article is smooth, i.e. the average peak to valley surface roughness is less than 0.2 micrometer.
Although the concept of raising the modulus of elasticity by incorporation of a second higher modulus phase is known, see Kingery et al. "Introduction to Ceramics", John Wiley & Sons, New York, N.Y. 1976, pages 773-777 (1976), it has not been proven practical for application to fibers or other sol-gel derived products having small dimensions. Commercially available, high modulus powders such as SiC, can be incorporated into these articles but the relatively large particle size (typically greater than 0.1 micrometer diameter and more typically greater than 1.0 micrometer diameter) leads to difficulties in spinning fibers, and more importantly leads to the formation of large flaws (voids, cracks, surface roughness) which negate any advantage which might be derived from the high modulus phase.
This invention provides ceramic articles having incorporated therein sufficient quantities of SiC such that the additive effect of the second phase can be achieved leading to a modulus of elasticity much higher than that inherent in the fully dense oxide ceramic.
U.S. Pat. No. 4,010,233 demonstrated improvements in the modulus of elasticity of alumina up to values of 269 GPa (39.times.10.sup.6 psi) using different dispersed phases to limit grain growth and minimize porosity. However, the improvements obtained are still well below the inherent modulus of elasticity of fully dense alumina [414 GPa (60.times.10.sup.6 psi)].
SiC derived from organic precursors may help control grain growth and porosity in oxide fibers and generally contains C and SiO.sub.2 which lower its effective modulus of elasticity to 207 GPa (.about.30.times.10.sup.6 psi). Thus SiC derived from such materials would not be expected to produce a significant increase in the moduli of oxides already having moduli of elasticity in this range. In contrast, higher purity forms of SiC have moduli of elasticity greater than 690 GPa (100.times.10.sup.6 psi) making such materials much more effective as an additive to produce a modulus increase above that which would be expected from the oxide itself.
In the present invention, the modulus of elasticity of fibers such as aluminum-borosilicates and zirconium silicates can be raised to values over 100 percent greater than that which could be obtained from the fully dense oxide fibers.
In this application:
"ceramic" means inorganic nonmetallic material consolidated by the action of heat, such as metal and nonmetal oxides;
"fully dense" means essentially free of pores or voids;
"sol" means a fluid solution or a colloidal suspension;
"non-vitreous" means not formed from a melt of the final oxide composition;
"green" refers to the ceramic articles which are unfired, that is, not in their ceramic form;
"amorphous" means a material having a diffuse X-ray diffraction pattern without definite lines to indicate the presence of a crystalline component;
"crystalline" means having a characteristic x-ray or electron diffraction pattern;
"dehydrative gelling" or "evaporative gelling" means that sufficient water and volatile material are removed from the shaped green fibers so that the form or shape of the fiber is sufficiently rigid to permit handling or processing without significant loss or distortion of the desired fibrous form or shape; all the water in the shaped fiber need not be removed. Thus, in a sense, this step can be called partial dehydrative gelling; and
"continuous fiber" means a fiber (or multi-fiber article such as a strand) which has a length which is infinite for practical purpose as compared to its diameter.